MS Calender: error message re Outlook when I don't have it! Why?

When I recently upgraded to Win 10, I decided to start using the Calendar app. I noticed at the time that whenever I added a new event, that 'calendar - outlook' appeared. That seemed to be the only option so I just ignored it. After a couple of weeks, I started getting a message that now pops up on my sys tray once an hour telling me that my 'Outlook account settings are out of date'.